It can be done but is going to require doing pretty much everything in code. The execution engine is currently modeled with the idea of interacting only with the most recent browser window opened. Under the covers the property Manager.ActiveBrowser is used by all standard (non-coded) test steps. This property is a read-only property. The framework will automatically set it to the most recently opened browser, allowing interaction with popup windows, and then automatically backtrack when the popup window is closed.
If you are willing to do things in code you need to start with something like this:
When a test starts one browser window is automatically opened by the framework. So the "Manager.LaunchNewBrowser" code will open a second browser window. Then using Manager.Browsers[n] you can select between which browser you want to interact with.
We can help you if you need further assistance with this approach.
the Telerik team